Wednesday, 13 January 2016
ATMS is an instrument for which high calibration accuracy in NWP applications is required. It is important for user to know the impact of antenna emissivity on the calibrated Earth observations. Since the radiation from cold space, warm load and the Earth scene are all collected through the polarized flat reflector, the impact of antenna emissivity on Earth scene includes two parts: 1) extra error will be introduced into calibration equation due to uncorrected antenna emissivity on calibration target, and 2) the calibrated earth scene brightness temperature will be contaminated directly by the reflector thermal radiation if not corrected. In our previous study, an error model has been developed for evaluation the impact of the first part, study results show that the calibration error decrease with increased scene temperature. To assess the impact of second part, full stokes vector RT model is required. The impacts of antenna emissivity to calibrated antenna temperature are complicated due to the rotation of polarization state. For polarized window channels, the scan angle dependent feature of TDR error is mainly dominated by the sin2θ term related to the third Stokes component of earth scene radiation, while for unpolarized channels the scan dependent feature of TDR error is dominated by sine square term at quasi-V channels and cosine square term in quasi-H channels. In this study, a full polarization RT model was developed to simulate the full stokes emissivity vector over ocean. Based on which the ATMS window channel observations can be simulated, and the impact of antenna emissivity on TDR products can be characterized.
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